1. Field of the Invention
The invention relates to a premixing burner, essentially comprising a pilot burner and a plurality of main burners arranged around the pilot burner.
2. Discussion of Background
Both in oil operation at very high pressure and in gas operation using gases containing a large amount of hydrogen, the ignition delay times in the case of premixing burners can be so short that flame-holding burners can no longer be used as so-called low-NO.sub.x burners.
The admixture of fuel to a combustion-air flow flowing in a premixing duct is generally performed by radial injection of the fuel into the duct by means of cross-jet mixers. However, the momentum of the fuel is so low that virtually complete mixing is achieved only after a distance of about 100 duct heights. Venturi mixers are also employed. The injection of the fuel via lattice arrangements is also known. Finally, injection ahead of special swirl-inducing bodies is also employed.
The devices operating on the basis of cross jets or laminar flows either result in very long mixing distances or require high injection momentums. In the case of premixing at high pressure and under substoichiometric mixing conditions, there is the risk of flashback of the flame or even self-ignition of the mixture. Flow separations and stagnation zones in the premixing tube, thick boundary layers on the walls or, in some cases, extreme velocity profiles across the cross section through which flow takes place can cause self-ignition in the tube or form paths by which the flame can flash back into the premixing tube from the combustion zone located downstream. Maximum attention must therefore be paid to the geometry of the premixing section.
The so-called premixing burners of the double-cone type may be referred to as flame-holding burners. Double-cone burners of this kind are known, for example, from U.S. Pat. No. 4,932,861 to Keller et al. and are described later with reference to FIGS. 1 and 3. The fuel, in that case natural gas, is injected in the inlet gaps into the combustion air flowing in from the compressor, via a row of injector nozzles. Generally speaking, these are distributed uniformly over the entire gap.
In order to achieve reliable ignition of the mixture in the downstream combustion chamber and sufficient burn-up, thorough mixing of the fuel with the air is required. Good mixing also contributes to the avoidance of so-called "hot spots" in the combustion chamber, these leading, inter alia, to the formation of the undesirable NO.sub.x.
The abovementioned injection of the fuel by conventional means such as, for example, cross-jet mixers is difficult since the fuel itself has an insufficient momentum to achieve the necessary large-scale distribution and fine-scale mixing.